Get 12 End Portal Frames
Find a flat 3×3 opening
Stand inside the empty center
Place 3 End Portal Frames on one side
Place 3 End Portal Frames on the opposite side
Place 3 End Portal Frames on the remaining two sides
Make sure all 12 frames form a complete square around the 3×3 center
Rotate each frame so the green eye socket faces inward
Insert an Eye of Ender into each frame
Fill all 12 frames with Eyes of Ender
The End Portal will activate automatically